Well, in that case, we have sent interstellar messages. At this moment, so far, there are 11 distinct human made things on trajectories out of the solar system into interstellar space. They're all related to five probes. Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 and New Horizons, the thing that recently made a Pluto flyby. These objects are our most distant hellos. Over the next ten thousand, million, billion years, they'll pass close enough to other star systems, maybe even planets, to possibly be discovered by other intelligent life forms. We had the foresight to include special messages on these probes.
Pioneer PlaqueThe Pioneer plaques are attached to Pioneer 10 and 11, which launched in the early 1970's, were the first human-made objects to ever be sent on a trajectory
to not just leave Earth, but to leave the solar system entirely. If discovered by other life out there, these plaques, designed by Frank Drake and Carl Sagan, could be our first chance to say "hello, we exist," or, depending on how long humanity lasts, our only chance to say "Hello, we existed. This is what we were." But will the plaques makes sense to aliens? Many human scientists have had trouble deciphering their meaning, but here's what they say. At the bottom is a map of our solar system with a path showing the Pioneer probe itself and where it came from. This element has been particularly criticized for being human centric. I mean, an arrow? Who's to say aliens will know that this depicts a path and not some structure in our solar system? Also, it's an arrow. Arrows might convey this way only two civilizations that hunted or developed pointy projectiles. Anyway. Up here, we define units. You can't tell aliens about humans or Earth by using seconds, kilometres or light years, because we made those measurements up. Instead, the plaque uses hyperfine transitions to communicate distances and time.
The hope is that curious intelligent life forms who find this will understand that this is a hydrogen atom - one proton, one electron. Hydrogen is the most abundant element in the universe, so hopefully its properties will be a common point of understanding. Now sometimes, if you've got enough hydrogen around, you can catch atoms in the heap transitioning between particles with parallel spins and antiparallel spins. Now, whenever this transition happens, electromagnetic radiation is released with a period of about 2.7 nanoseconds and a wavelength of about 21 centimetres. It's hoped that aliens equate this tick mark with these two units of measurement. For example, the woman is said to be, in binary, eight of these units tall. 21 centimetres times eight is a 168 centimetres. You would think that putting the probe itself behind them for scale would be enough to show our size, but having both might help solidify the connection.